External-beam radiotherapy for breast cancer is typically provided using opposing tangential fields, which deliver a uniform dose to the entire affected breast. The primary treatment is given over a number of sessions, and is often followed by additional boost sessions. The boost sessions are typically delivered with an electron beam, which limits treatment to the primary lumpectomy site.
Unlike photons, the intensities of which decrease exponentially as the photons travel through a patient, an electron beam deposits most of its energy dose within a fixed, finite range based primarily on the energy of the beam. Thus, a single electron beam can be used to treat superficial lesions while sparing underlying healthy tissues. Electron-beam treatments are typically delivered using electron cones of various sizes and shapes that may be attached to the collimator of a linear accelerator (LINAC), and which collimate the electron beam very close to the patient surface. The cones may have standard geometric shapes, such as circles or squares of various sizes, or an arbitrary shape can be custom-made for a given patient. In some instances, a lead sheet having an opening that defines the aperture of the beam is placed directly on the patient's skin.
Electron-beam treatment plans usually involve a fixed source-to-skin distance (SSD). For breast boosts, an SSD of 100 cm is typical, as this is the same distance from the beam source to the isocenter of most LINACS. As a result, the LINAC isocenter, and hence the intersection of any wall lasers being used to align the patient with the LINAC, lies on the surface of the patient's skin. This is in contrast to many photon treatments, which are planned such that the isocenter is near the center of the treatment volume within the patient, as opposed to on the patient's skin.
For a breast boost, the electron field ideally covers the tumor bed and the surgical path leading from the tumor bed to the surgical scar, plus a 1-2 cm margin. In addition, it is preferable to avoid the areola. Electron breast boosts may be simulated either using clinical or CT planning performed directly on the linear accelerator, or on a conventional simulator. In such simulations, a physician uses the lumpectomy scar and palpation to determine the location of the lumpectomy site relative to the patient's skin. A cut-out, usually made of CERROBEND, is designed to cover the region of interest on the patient's skin. The angles of the beam gantry and the couch on which the patient reclines are physically adjusted such that the beam is substantially perpendicular to (i.e., en face) the patient's skin. The appropriate electron energy is then chosen so that the beam covers the depth of the tumor bed, which may be found from post-surgery ultrasound scans, for example. The greater the energy of the electron beam, the deeper the electrons will penetrate. The correct number of “Monitor Units,” a calibrated measure of LINAC output, required to deliver a percentage of the prescribed dose at a given depth is calculated from tabulated beam data.
One weakness of clinical planning is that the actual position of the cavity is not explicitly taken into account. For this reason, in some institutions the simulation is performed using computed tomography (CT-SIM). One such technique uses radio-opaque wire placed around the surgical scar, and sometimes around the areola, prior to the acquisition of a CT scan. Thus the scar and the lumpectomy site, as seen on the CT scan, can be used to design the electron field. Energy and monitor units are calculated using treatment-planning software.
Once the plan (clinical or CT-based) is finished, the goal is to deliver radiation treatment according to the plan for each treatment session, or fraction thereof. For each fraction, the setup may be adjusted so that the field covers the same skin surface area as planned, using a preferred source-to-skin distance (SSD), with the beam oriented en face. These adjustments are often necessary because it is difficult to reposition the breast in exactly the same way from day to day since the breast is not a rigid structure, and consequently its shape, size and position can vary daily. Therefore, the setup can be adjusted by changing couch position, collimator angle, gantry angle and/or couch angle to take into account external features.
In making adjustments, it is possible to take into account not only external landmarks, but also internal anatomy. Physically moving and/or rotating the couch, gantry and collimator to properly orient the patient can be cumbersome, however, primarily due to the constraint of maintaining the planned SSD. Therefore, greater automation would be beneficial in utilizing internal and/or external landmark information to adjust patient setup, particularly where the planned SSD is taken into account.